The present invention relates to a multi-mode piezoelectric filter.
In recent years, a filter used in a MHz frequency range having a wide bandwidth of the passband and high temperature stability of frequency has been required for use in communication devices or the like. A multi-mode filter is generally employed in the MHz frequency range since it is able to reduce spurious response and is easy to manufacture. In general, this filter is made of material such as piezoelectric ceramics or quartz crystal.
When a multi-mode filter is made of piezoelectric ceramics, however, it is difficult to obtain high temperature stability, since its frequency greatly varies in relation to temperature in a practical temperature range (-20.degree. C. to 80.degree. C.).
On the other hand, a multi-mode filter made of quartz crystal has excellent temperature stability. However, it is difficult for this filter to function properly in communication devices since its bandwidth is too narrow.